This invention relates generally to fluid processing, and specifically to process flow measurement and control. In particular, the invention concerns measurement techniques for magnetic flowmeters.
Magnetic flowmeters (or mag meters) measure flow by Faraday induction, an electromagnetic effect. The meter energizes a coil to generate a magnetic field across a pipe section, and the magnetic field induces an electromotive force (EMF) across the process flow. The resulting potential difference (or voltage) is measured using a pair of electrodes that extend through the pipe section and into contact with the process flow, or via capacitive coupling. The flow velocity is proportional to the induced EMF, and the volumetric flow rate is proportional to the flow velocity and flow area.
In general, electromagnetic flow measurement techniques are applicable to water-based fluids, ionic solutions and other conducting flows. Specific uses include water treatment facilities, high-purity pharmaceutical manufacturing, hygienic food and beverage production, and chemical processing, including hazardous and corrosive process flows. Magnetic flowmeters are also employed in the hydrocarbon fuel industry, including hydraulic fracturing techniques utilizing abrasive and corrosive slurries, and in other hydrocarbon extraction and processing methods.
Magnetic flowmeters provide fast, accurate flow measurements in applications where differential pressure-based techniques are disfavored because of the associated pressure drop (for example, across an orifice plate or Venturi tube). Magnetic flowmeters can also be used when it is difficult or impractical to introduce a mechanical element into the process flow, such as turbine rotor, vortex-shedding element or Pitot tube.
Some magnetic flowmeters use field coils driven directly by AC line power. Another type of magnetic flowmeter, commonly referred to as a pulsed DC magnetic flowmeter, excites or powers the field coil periodically with a low frequency square wave. Pulsed DC magnetic flowmeters utilize a magnetic field which changes direction at a certain frequency. When the field is inverted, a spike in the electrode voltage is created by the rapidly changing magnetic field. This spike is not related to the flow rate, but to the rate of change of the magnetic field. In order to make a flow measurement, the electrode voltage measurement circuitry must be programmed to wait until this spike has completely decayed, or the voltage difference due to the decaying spike will appear to be measured flow.
The rate of decay of the voltage spike changes in relation to the impedance of the process fluid, which acts as a resistive-capacitive (RC) filter of the voltage spike. The impedance in turn depends on the conductivity of the process fluid, any coatings on the electrodes, and the length of the cable between the transmitter and the flow tube. Therefore, the rate of decay of the voltage spike is not known when the flowmeter leaves the factory and may change over time if the fluid conductivity changes or if coatings build up on the electrode surfaces.
Because the electrode-to-electrode impedance is not known in advance and may change over time, pulsed DC flowmeters are typically programmed to sample only at the very end of each half cycle where the probability of the voltage spike “leaking” into the flow measurement is very low. This measurement time may be, for example, only 20 percent of each half cycle.